nopCommerce 3.7 is old; I believe the version of reCaptcha that it uses is no longer available.
You could check the System > Log to see if you see a message to that effect. (Or maybe you need to look for warnings in the browser developer tools [F12] console).
Here's a SQL query I use on older systems to delete spammy customers. (Although I've not tried it with 3.70.)
As-is, it will
show you the suspect customers. Then you can remove the comment indicator "--" from the "--delete" line and add it to the "select *" line (i.e. make it "--select *"), and it will
delete the customers.
with del as
(
select Id, Email, CreatedOnUtc, Active, Deleted, LastIpAddress
from customer c
where CreatedOnUtc > GETUTCDATE()-1
and CreatedOnUtc = LastActivityDateUtc
and Email is not null
and not exists (select 1 from [Order] o where o.CustomerId = c.Id)
and exists (select 1 from GenericAttribute ca where ca.EntityId = c.Id and ca.[Key] = 'LastVisitedPage' and (ca.value like '%/register%'))
)
--delete customer where Id in (select Id from del)
select * from del
(you can adjust that GETUTCDATE()-1 as needed, and then create a SQL Server scheduled job to run the query periodcally)